Can non-destructive MRI provide a measurement of cartilage function? The delayed Gadolinium Enhanced MRI of Cartilage (dGEMRIC) index is highly correlated to mechanical stiffness, and, although not as good as a direct mechanical measurement, the index can be used as a predictive measure of stiffness.
Osteoarthritis (OA) involves a loss of balance between synthesis and breakdown of the constituents of articular cartilage. Glycosaminoglycans (GAGs) provide this cartilage its compressive strength and the dGEMRIC technique uses a negatively charged MR contrast agent to determine the GAG distribution within the cartilage.
Researchers from the New England Baptist Bone and Joint Institute and the Harvard MIT Division of Health Sciences and Technology (HST) in Boston used dGEMRIC MRI to determine if a relationship exists between the dGEMRIC index and indentation stiffness. (The dGEMRIC index is T1Gd, the MR T1 relaxation time measured in the presence of a contrast agent.) They explored whether a single relationship would adequately represent different cartilage conditions - specifically, whether the same relationship could describe different regions within the same joint (e.g. the region covered by the meniscus versus the region uncovered by the meniscus) and also describe cartilage obtained from cadaveric sources with no diagnosis of osteoarthritis versus cartilage from patients with osteoarthritis. They also determined the accuracy with which the dGEMRIC index can be used to predict local stiffness.
Twelve human tibial plateaus, six from cadavers with no clinical diagnosis of osteoarthritis and six from patients with osteoarthritis who were undergoing total joint replacement, formed the samples for the study. Each contained submensical ("covered") and extramensical ("uncovered") cartilage regions. The investigators used MR imaging to determine cartilage thickness and dGEMRIC index, then performed indentation testing. Finally, they performed statistical analyses to compare the compositions and mechanical responses of the two groups of samples.
Significant differences in both indentation stiffness and dGEMRIC index were found across regions, with regions covered by the meniscus being stiffer and having a higher index than uncovered regions. Despite the similar gross appearance of the tissue, the stiffness and dGEMRIC index in the cadaveric-derived cartilage were greater than for the surgically-derived tissue, suggesting that cartilage can be biomechanically abnormal, even when no overt sign of arthritic damage is evident.
The dGEMRIC index and indentation stiffness were highly correlated within regions, within cartilage from the same joint, and across cartilage from all the joints, so long as the dGEMRIC index from the region near the articular surface was used. If the average dGEMRIC index for full depth cartilage was used, the correlation was unreliable, suggesting that sufficient resolution for dGEMRIC imaging is needed if the data are to be used to reflect through thickness mechanical properties. The dGEMRIC index could be used to predict mechanical properties; however the prediction does not provide the sensitivity of a direct mechanical measurement.
The extent to which these results can be generalized is unknown. This study used grossly intact tissue from the tibial plateaus of subjects over the age of 50; so, it is not known yet whether these relationships and results would hold for a younger population, for cartilage from other joints, or for more grossly degenerated cartilage.
These data indicate that the dGEMRIC index can be used to infer stiffness in settings where the tissue is grossly intact and where the spatial resolution allows dGEMRIC measurement localized to the articular surface. While not as good as a direct mechanical measurement, because direct measurements are often impossible or undesirable, dGEMRIC can offer a reliable alternative.
These findings will be published in the April issue of the Journal of Orthopaedic Research, the official journal of the Orthopaedic Research Society.
Article: "Relationship between Cartilage Stiffness and dGEMRIC Index: Correlation and Prediction." Baldassarri, Marta, Little Goodwin, Jesse S., Farley, Michelle L., Bierbaum, Benjamin E., Goldring, Steven R.; Goldring, Mary B., Burstein, Deborah, Gray, Martha L., Journal of Orthopaedic Research, April 2007, (DOI: 10.1002/jor.20378).
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